Potential fixing device and potential fixing method

ABSTRACT

An electric potential fixing apparatus of the present invention is an electric potential fixing apparatus that is connected to a connection line ( 17 ) between two capacitances, the first capacitance ( 14 ) and the second capacitance ( 15 ) that is directly connected to the first capacitance, includes the first high resistance ( 3 ), the second high resistance ( 4 ) that is connected directly to the first high resistance, a voltage dividing unit that outputs electric potential divided by the first high resistance and the second high resistance to the output terminal, the third capacitance ( 8 ) that is connected in parallel to at least either of the first high resistance and the second high resistance, and a voltage supply unit ( 1 ) operable to maintain constantly electric potential of the connection line between the two capacitances ( 14 ) and ( 15 ), holding combined total electric charge quantity of the first capacitance and the second capacitance, and the output terminal of the voltage supply unit is connected to a signal line between the two capacitances.

TECHNICAL FIELD

The present invention relates to a capacitance measurement apparatus to measure a capacitance value of a capacitive sensor whose electrostatic capacitance changes in response to received physical quantity and particularly to the capacitance measurement apparatus equipped with an electric potential fixing/standard capacitance cancel circuit that fixes electric potential of a signal line in the capacitance measurement apparatus and cancels standard capacitance of a capacitive sensor.

BACKGROUND ART

Conventionally, a capacitance measurement apparatus that measures a capacitance value of a capacitive sensor such as a capacitor microphone whose electrostatic capacitance changes in response to received physical quantity (acceleration, pressure, gas, light, sound wave and so on) is known. FIG. 1 shows a conventional capacitance measurement apparatus 100. As shown in FIG. 1, the conventional capacitance measurement apparatus 100 includes an operational amplifier OP, an AC voltage generation apparatus OSC, a capacitive sensor C_(s), resistance R_(f) that is feedback impedance. The AC voltage generation apparatus OSC generates an operation signal V_(in) that is applied to the capacitive sensor C_(s) at the time of measuring the capacitance. The capacitive sensor C_(s) and an inversion input terminal of the operational amplifier OP are connected by the signal line L. The resistance R_(f) is connected between the signal line L and the operational amplifier OP. Additionally, the capacitive sensor C_(s) is connected between the inversion input terminal of the operational amplifier OP and the AC voltage generation apparatus OSC. One terminal of the AC voltage generation apparatus OSC is connected to standard electric potential.

As for an operation of the conventional capacitance measurement apparatus 100 shown in FIG. 1, when voltage V_(in) from the AC voltage generation apparatus OSC is supplied, alternating current flows to the capacitive sensor C_(s). In this case, since input impedance of the operational amplifier OP is ideally infinite, all the current that flows to the capacitive sensor C_(s) flows to the resistance R_(f).

Output of the capacitance measurement apparatus V_(out) can be derived from the following method.

When the amplitude of the operation signal is V, the angular velocity of the operation signal is ω_(in), the standard capacitance of the capacitive sensor is C_(d), the amplitude of change capacitance of the capacitive sensor C_(s) is C, and the angular velocity of capacitance change is ω_(c), the operation signal V_(in) and the capacitance of the capacitive sensor C_(s) can be represented by V _(in) =V sin ω_(in) t  (1) C _(s) =C _(d) +C sin ω_(c) t  (2)

Since the current I_(s) that flows through the capacitive sensor can be represented by I _(s) =d(C _(s) V _(in))/dt  (3) and the output V_(out) can be represented by V _(out) =−I _(s) R _(f)  (4), by the expressions (1) through (4) V _(out) =−R _(f){(C _(d) +C·sin ω_(c) t)·ω_(in)·cos ω_(in) t+C·ω _(c)·cos ω_(c) t·sin ω_(in) t}V  (5) is derived.

As is known from this expression (5), the output V_(out) has a term whose coefficient is the angular velocity of the capacitance change ω_(c). This means that in the case of the feedback impedance being the resistance, when the capacitance of the capacitive sensor changes at the frequency ω_(c), the output V_(out) that depends on the frequency ω_(c) is outputted (the output V_(out) has frequency dependence). Consequently, in the case of the feedback impedance being the resistance, a processing circuit that does not have a frequency characteristic in the subsequent stage must be configured, and therefore there is a problem that the size of the circuit becomes large.

There, technology that the feedback impedance is configured not by the resistance but by the capacitance is proposed. FIG. 2 shows the capacitance measurement apparatus 101 whose feedback impedance is configured by the capacitance C_(f). In this case, since the electric charge stored in the capacitive sensor C_(s) and that stored in the feedback capacitance C_(f.) are equal, −C _(f) ·V _(out) =C _(s) ·V _(in)  (6) holds, and therefore the output V_(out) can be represented by V _(out)=−(C _(d) +C sin ω_(c) t)/C _(f) ·V sin ω_(in)  (7)

As is known from this expression, output voltage V_(out) does not include a term that is proportional to the angular velocity ωc. This is because the electric charge of the signal line L that is connected to two capacitances is maintained constantly when the feedback impedance is configured by the capacitance.

As described above, since the term that is proportional to frequency of capacitance change dose not appear in circuit output, there is no need to set up a processing circuit newly in the subsequent stage. As a result, it is possible to prevent the size of the circuit from becoming large.

However, in the case of configuring the feedback impedance by the capacitance C_(f), the signal line L that connects C_(f) and the capacitive sensor becomes floating state electrically. For this reason, the electric potential of the signal line L becomes unstable and abnormality in a circuit operation that the circuit output is saturated with the power voltage may occur.

To prevent such a circuit abnormality, as shown in FIG. 2, it is conceivable to fix the electric potential of the signal line L by connecting resistance R_(g) between the signal line L and the ground.

However, in the case of fixing the electric potential by the resistance R_(g), at the time of measuring the capacitance, there may be a case that potential difference in the both terminals of the resistance R_(g) is generated and that current flows through the resistance R_(g). In that case, since the amount of electric charge varies in quantity, there is a problem that the sensibility of the capacitance measurement apparatus 101 decreases.

Consequently, it is desirable to propose a means to fix the electric potential of the signal line L without changing the electric charge quantity of the signal line L.

Additionally, when the standard capacitance C_(d) of the capacitive sensor C_(s) is very large compared with the capacitance change C, there is a problem that the capacitance change is not fully reflected in the output V_(out).

Consequently, even if the standard capacitance C_(d) is very large compared with the capacitance change C, a circuit with satisfactory sensibility is desirable.

The present invention is done to solve the above-mentioned problems and the object of the present invention is to provide a capacitance measurement apparatus equipped with an electric potential fixing means for fixing the electric potential level of the signal line without changing the electric charge quantity of the signal line of the capacitance measurement apparatus and a standard capacitance cancel means for canceling the effect that the standard (fixed) capacitance of the capacitive sensor has on the circuit output.

DISCLOSURE OF INVENTION

An electric potential fixing apparatus in one aspect of the present invention is an electric potential fixing apparatus that is connected to a connection line between two capacitances, a first capacitance and a second capacitance that is directly connected to the first capacitance comprising: an output terminal that is connected to the connection line; a voltage supply unit operable to maintain constantly electric potential of the connection line between the two capacitances, holding combined total electric charge quantity of the first capacitance and the second capacitance by supplying voltage to the connection line from the output terminal, wherein the voltage supply unit includes a first high resistance and a second high resistance that is directly connected to the first high resistance; a voltage dividing unit that outputs electric potential divided by the first high resistance and the second high resistance to the output terminal; and a third capacitance that is connected in parallel to at least either of the first high resistance and the second resistance.

Here, it is desirable to connect the first high resistance and the second high resistance in series. By the way, the high resistance in this invention can be realized using a reverse bias characteristic of diode and the off state of a transistor.

The electric potential fixing apparatus in the other aspect of this invention is an electric potential fixing apparatus that is connected to a connection line between two capacitances, a first capacitance and a second capacitance that is directly connected to the first capacitance comprising: an output terminal that is connected to the connection line; a voltage supply unit operable to output, to the connection line, electric potential that is equal to electric potential of an operation signal applied to the connection line; wherein the voltage supply unit includes a first high resistance and a second high resistance that is directly connected to the first high resistance; a voltage dividing unit that outputs electric potential divided by the first high resistance and the second high resistance to the output terminal; and a third capacitance that is connected in parallel to at least either of the first high resistance and the second high resistance.

Hereby, since entrance and exit of the electric current from the first high resistance and the second high resistance into the connection line is stopped, the electric charge of the connection line is maintained. In this way, since the electric charge of the connection line is maintained, for example, in a capacitance measurement apparatus, in the case of fixing the electric potential of the connection line of the first capacitance and the second capacitance, the sensibility of the capacitance measurement apparatus does not decrease. As a result, it is possible to execute an accurate capacitance measurement. Additionally, it is possible to easily adjust output electric potential of the voltage supply means by selecting appropriately the resistance values of the first high resistance and the second high resistance. By the way, it is desirable to connect the first high resistance and the second high resistance in series. Here, a high resistance means a resistance that has a sufficiently large and relatively high resistance value compared with the impedance components of the first capacitance and the second capacitance. Additionally, when the high resistance is viewed from a different aspect, it is possible to say that it has a nature that the input impedance of the electric potential fixing unit viewed from the connection line is larger than the input impedance to a circuit including either the first capacitance or the second capacitance viewed from the connection line.

Further, in the electric potential fixing apparatus in one aspect or the other aspect, it is desirable that the voltage supply unit further includes an amplifier and a predetermined voltage applying unit, the amplifier is connected to a terminal of the first high resistance, the other terminal of the first high resistance and a terminal of the second high resistance are connected, the output terminal is connected between the other terminal of the first high resistance and the terminal of the second high resistance, and the other terminal of the second high resistance and the predetermined voltage applying unit are connected each other.

When configured like this, by deciding the amplitude of the amplifier, the resistance values of the first fixed resistance and the second fixed resistance, and the voltage value of the predetermined electric potential applying unit, it is possible to easily control the electric potential of the output terminal of the voltage supply unit to be the same electric potential as the electric potential of the operation signal applied to the connection line between the first capacitance and the second capacitance. Furthermore, by deciding the amplification factor of the amplifier, the capacitance value of the capacitance to cancel the standard capacitance, it is possible to easily control the supply quantity from the capacitance to cancel the standard capacitance out of the electric current that flows through the first capacitance.

The electric potential fixing apparatus in one aspect or the other aspect further comprises a first operational amplifier, the first capacitance is a measuring capacitance, the connection line between the two capacitances is a signal line, and the input terminal of the first operational amplifier is connected to the signal line.

Additionally, the electric potential fixing apparatus in one aspect or the other aspect further comprises the second operational amplifier, and the output terminal of the second operational amplifier is connected to the second capacitance.

An electric potential fixing method in one aspect of this invention is an electric potential fixing method for fixing electric potential of a connection line between two capacitances, a first capacitance and a second capacitance that is directly connected to the first capacitance, using a voltage supply unit which has an amplifier, a third capacitance, and at least two high impedances that divide the voltage, the third capacitance being connected with at lest one of the high impedances, the electric potential fixing method including: a step for applying output of the voltage supply unit to the connection line between the two capacitances; and a step for deciding fixed electric potential of a connection line by adjusting an amplitude of the amplifier and a capacitance value of the third capacitance.

Note that the high impedance and the high resistance have the same function.

The electric potential fixing method in the other aspect of this invention is an electric potential fixing method for fixing electric potential of a connection line between two capacitances, a first capacitance and a second capacitance that is directly connected to the first capacitance, using a voltage supply unit which has an amplifier, a first high resistance and a second high resistance, and a third capacitance that is connected in parallel to at least either of the first high resistance and the second high resistance, the electric potential fixing method including; a step for outputting electric potential divided by the first high resistance and the second high resistance to the connection line between the two capacitances; a step for setting up electric potential of an operation signal that is applied to the connection line between the two capacitances and output electric potential of the voltage supply unit to become equal by adjusting an amplitude of the amplifier, values of the two high resistances and a value of a third capacitance.

It is desirable that in the electric potential fixing method in one aspect or the other aspect of this invention, a terminal part of either the first high resistance or the second high resistance of the voltage supply unit includes a predetermined voltage applying unit, and the electric potential of an operation signal that is applied to the connection line between the two capacitances and the output electric potential of the voltage supply unit are set up to become equal by adjusting applied voltage of the predetermined voltage applying unit.

Additionally, the electric potential fixing method in one aspect or the other aspect uses either the first capacitance or the second capacitance as a measuring capacitance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram that shows a conventional capacitance measurement apparatus.

FIG. 2 is a circuit diagram that shows an electric potential fixing method as a comparative example.

FIG. 3 is a circuit diagram that shows a capacitance measurement apparatus including an electric potential fixing apparatus according to the first embodiment of the present invention.

FIG. 4 is a circuit diagram that shows an example of an internal configuration of an amplifier included in the electric potential fixing apparatus according to the first embodiment shown in FIG. 3.

FIG. 5 is a circuit diagram that shows a capacitance measurement apparatus including an electric potential fixing apparatus according to the second embodiment of the present invention.

FIG. 6 is a circuit diagram that shows a capacitance measurement apparatus including an electric potential fixing apparatus according to another embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 3 is a circuit diagram that shows a capacitance measurement apparatus including an electric potential fixing apparatus according to the first embodiment of the present invention.

For a start, referring to FIG. 3, the configuration of the capacitance measurement apparatus including the electric potential fixing apparatus according to the first embodiment is explained. The capacitance measurement apparatus according to the first embodiment includes an operational amplifying circuit 11 that is a voltage generator to acquire a gain, an operational amplifier 12 in a state of imaginary short, an AC voltage generator 13, a measuring capacitance 14, and a feedback capacitance 15. By the way, the operational amplifier 12 is an example of “the first operational amplifier” of the present invention and the operational amplifying circuit 11 is an example of “the second operational amplifier” of the present invention. Additionally, the measuring capacitance 14 is an example of “the first capacitance” or “the second capacitance” of the present invention and its capacitance value C_(s) is represented by an addition of the standard capacitance C_(d) and the change capacitance C sin ω_(c)t, namely, C_(s)=C_(d)+C sin ω_(c)t. The standard capacitance is the capacitance that the first capacitance and the second capacitance own characteristically and the fixed capacitance value before external force is added. The feedback capacitance 15 is an example of “the first capacitance” or “the second capacitance” of the present invention. The measuring capacitance 14 and the feedback capacitance 15 are connected by a signal line 17. In addition, this signal line 17 is an example of “a connection line” of the present invention. The terminal of the detecting capacitance 14 that is not connected to the signal line may be in a state of floating but it is possible to measure with high accuracy when the end of the measuring capacitance 14 is connected to the predetermined electric potential. The signal line 17 is connected to an input terminal of the operational amplifier 12. Furthermore, the AC voltage generator 13 is connected to the other input terminal of the operational amplifier 12.

Here, in the first embodiment, the electric potential of the signal line 17 is fixed using the electric potential fixing apparatus equipped with a voltage supply circuit 1. Note that the voltage supply circuit 1 is an example of “a voltage supply means” of the present invention. The voltage supply circuit 1 includes an amplifier 2 that has an amplitude A, the first high resistance 3 that has a resistance value of R_(a1), the second high resistance 4 that has a resistance value of R_(a2), and a capacitance to cancel the standard capacitance 8 that has a capacitance value of C_(c). It is acceptable if R_(a1) of the first high resistance 3 and R_(a2) of the second high resistance are the resistance values that have sufficiently high values relatively compared with an approximate characteristic impedance value obtained by the frequency and the detected capacitance used together. By the way, a variant A that is indicated by A times and so on in the present invention is a real number except 0.

Moreover, to the input side of the amplifier 2 is connected a different AC voltage generator (another power) 7 from the AC voltage generator 13. To the output side of the amplifier 2 is connected one terminal of the first high resistance 3. Between the other terminal of the first high resistance 3 and one terminal of the second high resistance 4 is connected an output terminal 5. The output terminal 5 of the voltage supply circuit 1 is connected to the signal line 17 at the point P. A terminal 6 is set up to the other terminal of the second high resistance 4. To the terminal 6 is applied the predetermined electric potential V_(s) This terminal 6 is an example of “the predetermined voltage applying means”. Additionally, from the output terminal 5 is outputted Voltage Va divided by the resistance partition of the first high resistance 3 and the second high resistance 4.

Additionally, the amplifier 2, for example, has a configuration shown in FIG. 4. Namely, the amplifier 2 includes an operational amplifier 21, a resistance 22 that has a resistance value of R₁, and a resistance 23 that has a resistance value of R₂. To the non-inverting input terminal of the operational amplifier 21 is connected the AC voltage generator 7 (refer to FIG. 3). Furthermore, between the output terminal and the non-inverting input terminal of the operational amplifier 21 is connected the resistance 22. Moreover, between the non-inverting input terminal of the operational amplifier 21 and GND is connected the resistance 23. When configured like this, it is possible to obtain easily the amplifier 2 that has an amplitude A=(R₁+R₂)/R₂.

As for the electric potential fixing method of the capacitance measuring apparatus according to the first embodiment, the amplitude A of the amplifier 2, the resistance R_(a1) of the first high resistance 3, the resistance R_(a2) of the second high resistance 4, and the voltage V of the terminal 6 V_(s) are decided in order that the voltage V_(in) of the operation signal that flows through the signal line 17 and the voltage V_(a) of the output terminal 5 of the voltage supply circuit 1 become equal. The capacitance value C_(c) of the capacitance to cancel the standard capacitance 8 as the third capacitance and the amplitude A of the amplifier 2 are decided in order to supply at least part of the current that flows through the measuring capacitance 14.

As for the capacitance measurement operation of the capacitance measurement apparatus of the first embodiment shown in FIG. 3, since the operational amplifier 12 is in the state of imaginary short, the voltage V_(in) (of the operation signal) from the AC voltage generator 13 is applied to the signal line 17. By so doing, the voltage is applied to the both terminals of the measuring capacitance 14 and the current flows. Then, the output V_(out) that corresponds to electrostatic capacitance C_(s) of the measuring capacitance 14 is outputted from the signal output terminal 18. By executing various kinds of signal processing on this output voltage V_(out), the electrostatic capacitance C_(s) of the measuring capacitance 14 is obtained.

In the first embodiment, as is described above, the voltage supply circuit 1 that applies the alternating current in order to fix the electric potential and cancel the standard capacitance to the signal line 17 that connects the measuring capacitance 14 and fixed capacitance 15, by setting up the amplitudes A, R_(a1), and R_(a2) of the amplifier 2 in order that the electric potential of the output terminal 5 of the voltage supply circuit 1 determined by dividing pressure of the output of the amplifier 2 with the first high resistance 3 and the second resistance 4 becomes equal to the electric potential V_(in) of the operation signal that is applied to the signal line 17, entrance and exit of the electric current from the first high resistance 3 and the second high resistance 4 into the signal line 17 is stopped and the electric charge quantity is prevented from changing. Additionally, by including the first high resistance 3 and the second high resistance 4 that have high impedance values in the voltage supply circuit 1, it is possible to effectively prevent part of the electric current that flows through the signal line 17 from flowing into the voltage supply circuit 1. This also can prevent the electric charge quantity of the signal line 17 from changing. In addition, by supplying at least a part of electric current that flows through the measuring capacitance 14 from the amplifier 2 through the capacitance to cancel the standard capacitance 8, the electric current supplied from the operational amplifying circuit 11 through the feedback capacitance 15 to the standard capacitance of the measuring capacitance decreases and capacitance changing part of the measuring capacitance is sufficiently reflected in output V_(out). Furthermore, by supplying the electric current to the measuring capacitance from the amplifier 2 through the capacitance to cancel the standard capacitance, the electric current that flows through the feedback capacitance 15 decreases, in other words, the offset part of the electric current that flows through the feedback capacitance decreases and therefore it is possible to raise the gain of the capacitance measurement apparatus decided by C_(f).

As a result, in the capacitance measurement apparatus according to the first embodiment, even in the case of fixing the electric potential of the signal line 17 that connects the measuring capacitance 14 and the fixed capacitance 15, since the sensibility of the capacitance measurement apparatus does not decrease, it is possible to execute an accurate capacitance measurement.

The first embodiment is explained below using expressions.

For a start, the output V_(a) of the voltage supply circuit can be represented by V _(a) =R _(a2)(AV _(o) −V _(s))/(R _(a1) +R _(a2))  (8)

Here, assume that A=2, V_(a)=0 and R_(a1)=R_(a2) for simplicity. At this time, since it is set up to be V_(a)=V_(in) (V_(a) and V_(in) are same electric potential), it is all right to be V_(o)=V_(in)  (9)

Consequently, the electric current I_(c) that flows through the capacitance to cancel the standard capacitance C_(c) can be represented by I _(c) =d{C _(c)(AV _(o) −V _(in))}/dt=d(C _(c) V _(in))/dt  (10)

Next, since the operational amplifying circuit 12 is in the state of imaginary short, the voltage V_(in) (the operation signal) from the altering voltage generator 13 is applied to the signal line 17. By so doing, the electric current flows through the detecting capacitance 14. This electric current I_(a) can be represented by I _(s) =d(C _(s) V _(in))/dt=(C _(d)ω_(in) cos ω_(in) t+Cω _(c) cos ω_(c) t·sin ω_(in) t+C ω _(in) sin ω_(c) t·cos ω_(in) t)V  (11)

Here, C_(d) is the standard capacitance value of the detecting capacitance 14, C is the amplitude of change capacitance of the measuring capacitance 14, ω_(c) is the angular frequency of capacitance change, V is the amplitude of the operation signal, and ω_(in) is the angular frequency of the operation signal.

Moreover, the electric current I_(f) that flows through the feedback capacitance C f can be represented by I _(f) =d{C _(f)(V _(out) −V _(in))}/dt  (12)

Here, since the input impedance of the operational amplifying circuit 12 is sufficiently high, and it is set up to be V_(a)=V_(in), and R_(a1) and R_(a2) are sufficiently high resistance values, the electric current does not flow through R_(a1) and R_(a2) and therefore I_(c), I_(s), and I_(f) have the following relationship: I _(f) =I _(s) −I _(c)  (13) d{C _(f)(V _(out) V _(in))}/dt=d(C _(s) V _(in))/dt−d(C _(c) V _(in))/dt  (14)

Consequently, the output V_(out) of the signal output terminal 18 is V _(out)={1+(C _(s) −C _(c))/C _(f) }V _(in)={1+(C _(d) +C sin ω_(c) t−C _(c))/C _(f) }V sin ω_(in) t  (15)

Under these conditions (A=2, V_(s)=0 and R_(a1)=R_(a2)), by making C_(c) equal to C_(d), it is possible to preclude the influence of the standard capacitance of the detecting capacitance 14 to circuit output. In other words, out of the detecting capacitance 14 only the signal that corresponds to the change capacitance is outputted to the circuit output. In sum, in this case, since the circuit output can be represented by V _(out)=(1+C sin ω_(c) t/C _(f))V sin ω_(in) t  (16), the standard capacitance does not influence the circuit output and therefore it is possible to measure the capacitance change C accurately with high sensibility.

In the foregoing, explanation is made focusing on the attention on the electric current; the confirmation is given below focusing on the voltage.

Since the input impedance of the operational amplifying circuit 12 is sufficiently high, and it is set up to be V_(a)=V_(in), and R_(a1) and R_(a2) are sufficiently high resistance values, the electric current does not flow through R_(a1) and R_(a2) and therefore the electric charge quantity of the signal line is constant. Consequently, C _(c) V _(in) +C _(f)(V _(out) −V _(in))=C _(s) V _(in)  (17) V _(out)={1+(C _(s) −C _(c))/C _(f) }V _(in)={1+(C _(d) +C sin ω_(c) t−C _(c))/C _(f) }V sin ω_(in) t  (18)

This is same as the above-mentioned expression of V_(out). Consequently, in the embodiment shown in FIG. 3, under the above-mentioned conditions, it is possible to execute a correct capacitance measurement.

FIG. 5 is a circuit diagram that shows a capacitance measurement apparatus equipped with an electric fixing/standard capacitance cancel means including a voltage supply circuit according to the second embodiment of the present invention. In this voltage supply circuit 1 according to the second embodiment, the AC voltage generator 13 to apply the operation signal V_(in) to the signal line 17 instead of the AC voltage generator 7 is connected to the input side of the amplifier 2 in the configuration according to the above-mentioned first embodiment. By the way, the other configuration according to the second embodiment is similar to the first embodiment.

In the second embodiment, as is described above, by connecting the AC voltage generator 13 to apply the operation signal V_(in) to the signal line 17 with the input side of the amplifier 2, it is possible to omit the AC voltage generator 7 according to the first embodiment, and therefore it is possible to simplify the circuit configuration compared with the first embodiment.

Additionally, in the second embodiment, similarly to the above-mentioned first embodiment, by adjusting the amplitude A of the amplifier 2, the resistance value R_(a1) of the first high resistance 3, the resistance value R_(a2) of the second high resistance 4, and the voltage V_(s) of the terminal 6, it is possible to easily set up the voltage V_(a) in the output terminal 5 of the voltage supply circuit 1 to be equal to the voltage V_(in) of the operation signal of the signal line 17. Additionally, by adjusting the amplitude A of the amplifier 2 and the capacitance value C_(c) of the capacitance to cancel the standard capacitance, it is possible to set up cancel quantity of the measuring capacitance. More specifically, it is possible to cause the amplitude A to be A=2 by making the resistance values of resistance 22 and 23 of the amplifier 2 shown in FIG. 4 R₁'R_(2.); it is possible to cause easily the voltage Va of the output terminal 5 of the voltage supply circuit 1 to be equal electric potential to the voltage V_(in) by making V_(s)=0 V, and R_(a1)=R_(a2). Furthermore, when the standard capacitance of the detecting capacitance 14 is C_(c)=C_(d) and the amplitude of the amplifier 2 is A=2, as described above, all the electric current that flows through the standard capacitance in the detecting capacitance is supplied by the standard capacitance cancel capacitance, only the electric current that flows through the change capacitance component flows through the feedback capacitance 15 and therefore the standard capacitance value does not influence the output V_(out).

In addition, the embodiments disclosed this time should be thought to be exemplification in all respects and not to be limited. The scope of the present invention is shown not by the above-described explanation of the embodiments but by the scope of claims and further all changes in the scope of claims and in the meaning and scope of uniformity are included.

For example, in the above-described embodiments, the first high resistance 3 and the second high resistance 4 are used as the high impedance of the voltage supply circuit 1, but the present invention is not limited to this. It is acceptable, for example, to use the reverse bias characteristic of diode as the high impedance and it is possible to use an off state of a transistor. In other words, the high impedance and the high resistance have the same function to work as the resistance component.

In FIG. 3 and FIG. 5, the circuit is configured with two operational amplifying circuits but it is acceptable to configure the circuit with one operational amplifying circuit as in FIG. 6. Furthermore, 12 is configured with the operational amplifying circuit but it is acceptable to configure 12 with an impedance exchange.

Moreover, in the above-described embodiments, the capacitance measurement apparatus with the circuit configuration shown in FIG. 3 and FIG. 5 is explained, but the present invention is not limited to this and similarly applicable to a capacitance measurement apparatus with another circuit configuration.

Additionally, in the above-described embodiments, a case of fixing the electric potential of the signal line 17 that connects the measuring capacitance 14 and the fixed capacitance 15 in the capacitance measurement apparatus is explained, but the present invention is not limited to this and widely applicable to a case of fixing the electric potential of a apparatus other than the capacitance measurement apparatus including a circuit configuration where the first capacitance and the second capacitance are directly connected.

As is described above, by the present invention, in the case of fixing the electric potential of the connection line between the first capacitance and the second capacitance, it is possible to prevent the electric charge quantity of the connection line between the first capacitance and the second capacitance from changing. Furthermore, since at least part of the electric current that flows through the measuring capacitance can be supplied through the capacitance to cancel the standard capacitance, it is possible to sufficiently cover the capacitance change that takes place in the output V_(out). As a result, for example, in the case of fixing the electric potential of the connection line between the first capacitance and the second capacitance in the capacitance measurement apparatus, the sensibility of the capacitance measurement apparatus does not decrease and therefore it is possible to execute an accurate capacitance measurement with high sensibility.

Industrial Applicability

The electric potential fixing apparatus according to the present invention can be used as a capacitance measurement apparatus and a capacitance detection apparatus to measure the capacitance value of the capacitive sensor whose electrostatic capacitance changes in response to the received physical quantity and particularly as the capacitance measurement apparatus equipped with the electric potential fixing/standard capacitance cancel circuit that cancels the standard capacitance of the capacitive sensor and as the electric potential fixing circuit for a microphone apparatus included in a small and light apparatus such as a cell phone. 

1. An electric potential fixing apparatus that is connected to a connection line between two capacitances, a first capacitance that is a measuring capacitance and a second capacitance that is directly connected to the first capacitance comprising: an output terminal that is connected to the connection line; a voltage supply unit operable to maintain constantly electric potential of the connection line between the two capacitances, holding combined total electric charge quantity of the first capacitance and the second capacitance by supplying voltage to the connection line from the output terminal; an alternating-current voltage generator which generates an alternating-current voltage; and a first operational amplifier having an input terminal connected to the connection line and another input terminal connected to the alternating-current voltage generator; wherein the voltage supply unit includes a first high resistance and a second high resistance that is directly connected to the first high resistance; a voltage dividing unit that outputs electric potential divided by the first high resistance and the second high resistance to the output terminal; and a third capacitance that is connected in parallel to at least either of the first high resistance and the second high resistance.
 2. An electric potential fixing apparatus that is connected to a connection line between two capacitances, a first capacitance that is a measuring capacitance and a second capacitance that is directly connected to the first capacitance comprising: an output terminal that is connected to the connection line; a voltage supply unit operable to output, to the connection line, electric potential that is equal to electric potential of an operation signal applied to the connection line; an alternating-current voltage generator which generates an alternating-current voltage; and a first operational amplifier having an input terminal connected to the connection line and another input terminal connected to the alternating-current voltage generator; wherein the voltage supply unit includes a first high resistance and a second high resistance that is directly connected to the first high resistance; a voltage dividing unit that outputs electric potential divided by the first high resistance and the second high resistance to the output terminal; and a third capacitance that is connected in parallel to at least either of the first high resistance and the second high resistance.
 3. The electric potential fixing apparatus according to claim 1 or claim 2, wherein the voltage supply unit further includes an amplifier and a predetermined voltage applying unit, the amplifier is connected to a terminal of the first high resistance, the other terminal of the first high resistance and a terminal of the second high resistance are connected, the output terminal is connected between the other terminal of the first high resistance and the terminal of the second high resistance, and the other terminal of the second high resistance and the predetermined voltage applying unit are connected each other.
 4. The capacitance measurement apparatus according to claim 1 or claim 2, wherein the electric potential fixing apparatus further comprises the second operational amplifier, and the output terminal of the second operational amplifier is connected to the second capacitance.
 5. An electric potential fixing method for fixing electric potential of a connection line between two capacitances, a first capacitance and a second capacitance that is directly connected to the first capacitance, using a voltage supply unit which has an amplifier, a first high resistance and a second high resistance, and a third capacitance that is connected in parallel to at least either of the first high resistance and the second high resistance, the electric potential fixing method including; a step for outputting electric potential divided by the first high resistance and the second high resistance to the connection line between the two capacitances; a step for setting up electric potential of an operation signal that is applied to the connection line between the two capacitances and output electric potential of the voltage supply unit to become equal by adjusting an amplitude of the amplifier, values of the two high resistances and a value of a third capacitance.
 6. The electric potential fixing method according to claim 5, wherein a terminal part of either the first high resistance or the second high resistance of the voltage supply unit includes a predetermined voltage applying unit, and the electric potential of an operation signal that is applied to the connection line between the two capacitances and the output electric potential of the voltage supply unit are set up to become equal by adjusting applied voltage of the predetermined voltage applying unit.
 7. The electric potential fixing method according to claim 5, wherein either the first capacitance or the second capacitance is used as a measuring capacitance. 